1. Field of the Invention
The present invention relates to ion conductive polymers and imide monomers. More specifically, the present invention relates to ion conductive polymers suitable as electrolyte membranes for use in a variety of electrochemical devices, including fuel cells, secondary batteries, water electrolyzers, hydrohalogenic acid electrolyzers and sodium chloride electrolyzers, oxygen and/or hydrogen concentrators, humidity sensors, gas sensors, and the like, as proton conductors for use in electrolytes in the catalyst layers or as lithium ion conductors for use in lithium ion batteries, and the like, and further relates to imide monomers for the preparation of the ion conductive polymers.
2. Description of the Related Art
Solid polymer electrolytes are solid polymeric materials that have ion exchange groups, e.g., sulfonic acid groups, in their polymer chains, and function as ionic conductors for transporting ions, such as protons and lithium ions.
For example, solid polymer electrolytes for use in a variety of electrochemical devices, including solid polymer fuel cells, water electrolyzers, etc., are shaped into a membrane and are used in the form of a membrane-electrode assembly (MEA) wherein a pair of electrodes are bonded to both surfaces of the electrolyte membrane. General electrodes for use in solid polymer fuel cells have a bilayer structure consisting of a diffusion layer and a catalyst layer. The diffusion layer is a layer for supplying a reactive gas and electrons to the catalyst layer, and is made of a carbon fiber, a carbon paper, or the like. The catalyst layer is a portion where an electrode reaction is taken place, and is commonly composed of a composite of an electrode catalyst and a solid polymer electrolyte.
Fluorine-based electrolytes, typified by Nafion (registered trademark for products manufactured by DuPont), and hydrocarbon-based electrolytes have been known as solid polymer electrolytes for use in a variety of electrochemical devices. Since perfluorinated electrolytes have superior durability over hydrocarbon-based electrolytes, they are widely used in water electrolyzers, fuel cells, and other devices. However, perfluorinated electrolytes, e.g., Nafion (registered trademark), are highly priced. And, solid polymer electrolytes for use in electrochemical devices are required to have better performance in order to improve the performance of the electrochemical devices using the solid polymer electrolytes.
Various proposals have been made to solve these problems. For example, Patent Publication 1 discloses an ion conductive polymer which has a main chain of a hydrocarbon-based polymer, such as a polyphenylene backbone, a polyester backbone, etc. And the ion conductive polymer has such structure that a protonic acid group, such as a sulfonic acid group, etc., is separated from the main chain by a spacer structure formed by an alkyl or aryl group, etc. In addition, Patent Publication 1 describes that the presence of the spacer structure separating the protonic acid group from the main chain allows the ion conductive polymer to exhibit superior ion conducting properties and high heat resistance.
Further, Patent Publication 2 discloses an ion conductive polymer membrane for a fuel cell which is composed of polyamide containing a protonic acid group, such as a sulfonic acid group, etc. Further, Patent Publication 3 discloses an ion conductive polymer membrane that is composed of a mixture of a protonic acid group-containing polyamide with a halogenated hydrocarbon resin, a polyfluorovinylidene resin or the like. Patent Publication 2 describes that the polymer membrane composed of a protonic acid group-containing polyamide exhibits excellent film-forming ability, superior ion conducting properties and high heat resistance.
Further, Patent Publication 4 discloses a polymer prepared by homopolymerization of lithium N-(trifluoromethanesulfonyl)-2-(4-ethenylphenoxy) tetrafluoroethane sulfone imide (CH2═CH—C6H4—O—CF2CF2SO2N(Li)SO2CF3). Patent Publication 4 describes that the styrene-based polymer exhibits high solubility in various kinds of organic solvents, making it easy to shape, and it can be prepared at low costs due to the low content of fluorine.
Further, Patent Publication 5 discloses a fluorine-based copolymer-containing composition which includes a perfluorinated copolymer including —CF2CF2—, —CF2CF(—O—(CF2CF(CF3)—O)n—(CF2)m—SO2F)— and —CF2CF(—O—(CF2CF(CF3)—O)n′—(CF2)m′—SO2NHR)— as repeating units, and a liquid fluorooligo ether consisting of one or more kinds of a repeating unit represented by —(Rf—O)—. Patent Publication 5 describes that a sulfone imide crosslinked structure can be introduced into the perfluorinated copolymer by reacting a SO2F type group with a SO2NHR type group in the presence of a Lewis base, and the introduction of the crosslinked structure results in improved durability.
Non-Patent Publication 1 discloses trisodium bis[(perfluoroalkyl) sulfonyl]triimide (C4F9—SO2N−Na+SO2N−Na+SO2N−Na+SO2C4F9) prepared by reaction of C4F9SO2NHNa and HN(SO2Cl)2. Non-Patent Publication 1 describes that the trisodium salt is more electrochemically stable than NaHFPSI (((CF3)2CHO—SO2)2NNa) and NaTFSI ((CF3SO2)2NNa) and that a solution (0.01 M) of the trisodium salt in DMF exhibits a higher electrical conductivity than a solution (0.01M) of NaHFPSI or NaTFSI in DMF.
[Patent Publication 1] Japanese Patent Unexamined Publication No. 2002-289222, Paragraph No. [0007] and [0022]-[0026]
[Patent Publication 2] Japanese Patent Unexamined Publication No. 2002-280019, claim 1, Paragraph No. [0009]
[Patent Publication 3] Japanese Patent Unexamined Publication No. 2003-109624, claim 3, Paragraph No. [0072]
[Patent Publication 4] Japanese Patent Unexamined Publication No. 2003-525957, Paragraph No. [0032] and
[Patent Publication 5] Japanese Patent Unexamined Publication No. 2003-246906, claim 1, Paragraph No. [0057]
[Non-Patent Publication 1] “Synthesis and characterization of a Novel electrolyte based on bis(perfluoroalkyl)sulfonyl)triimide trianion”, J. Nie et al., J. Fluorine Chemistry, 125 (2004) 27-31
The solid polymer electrolytes disclosed in Patent Publications 1 to 4 have C—H bonds within their molecules and show poor resistance to peroxide radicals. Accordingly, if the solid polymer electrolytes are used in the production of separators for water electrolyzers and electrolyte membranes for fuel cells, the separators and membranes are degraded by peroxide radicals and thus high durability is not attained. Perfluorinated electrolytes having excellent oxidation resistance and high durability are commonly used in the production of the membranes and separators. However, the membranes and separators produced using perfluorinated electrolytes may be degraded under extreme conditions of use. For these reasons, solid polymer electrolytes used in the production of membranes and separators are required to have excellent oxidation resistance and high durability.
The performance of electrochemical devices is dependent on that of solid polymer electrolytes used in the fabrication of the devices. Generally, the performance of electrochemical devices is improved as the proton conductivity of solid polymer electrolytes increases. To increase the proton conductivity of solid polymer electrolytes, methods for increasing the number of acid groups (e.g., sulfonic acid groups) present within the solid polymer electrolytes are commonly employed. However, swelling or dissolution of solid polymer electrolytes in water tends to increase with increasing number of acid groups present in the solid polymer electrolytes. For this reason, solid polymer electrolytes having high strength and excellent heat resistance are not attained by the methods.
To improve the efficiency of fuel cells, it is preferred to increase the operating temperature of the fuel cells. To this end, the use of solid polymer electrolytes having high strength and excellent heat resistance is desirable. However, since conventional perfluorinated electrolytes, typified by Nafion, are not crosslinked and have low crystallinity, they have problems of poor strength and heat resistance.
The sulfone imide group (—SO2NHSO2—) has a relatively high proton conductivity and is highly resistant to radicals. As taught in Patent Publication 5, the introduction of a sulfone imide crosslinked structure into a membrane by reaction of a SO2F type group with a SO2NHR type group leads to an improvement in the strength and heat resistance of the membrane.
However, only one proton in the sulfone imide group is effective. If the number of sulfone imide groups introduced is increased to increase the number of effective protons in an electrolyte, the resistance to swelling and resistance to dissolution in water of the electrolyte is deteriorated. In addition, the hydrophilicity of the sulfone imide group is insufficient, which makes it difficult to form effective ion channels in the electrolyte. Furthermore, the introduction of sulfone imide groups by crosslinking causes consumption of acid groups.
Accordingly, there is a limitation in simultaneously achieving high strength, excellent heat resistance and high proton conductivity in conventional electrolytes containing sulfone imide groups.
Further, the sodium salt of the trimide anion disclosed in Non-Patent Publication 1 has a low molecular weight and has no film-forming properties. In addition, there has been made no attempt to apply the trimide anion to a solid polymer electrolyte in order to improve both strength and ionic conductivity of the solid polymer electrolyte.